C (programming language)

C program to print following numbers series 1 2 3 6 9 18 27 54 81

Understanding the Number Series

  • This number series doesn’t follow a standard mathematical progression like arithmetic or geometric sequences.
  • It appears to have a pattern of alternating multiplication and addition operations.
  • To generate this series in C, we’ll create a custom logic that replicates this pattern.

Approach to Printing the Series

  1. Initialize variables:
    • Start with the first two numbers (1 and 2) stored in separate variables.
  2. Use a loop:
    • Employ a loop to iterate through the desired number of terms.
  3. Apply the pattern:
    • Inside the loop, perform the following actions:
      • Print the current value of the first variable.
      • Calculate the next value by multiplying the second variable by 3.
      • Print the current value of the second variable.
      • Calculate the next value by adding 3 to the first variable.
  4. Update variables:
    • Assign the newly calculated values to the respective variables for the next iteration.

C Program Implementation


#include <stdio.h>

int main() {
    int num1 = 1, num2 = 2;
    int n = 9;  // Number of terms to print

    printf("Number Series:\n");

    for (int i = 1; i <= n; i++) {
        printf("%d ", num1);      // Print the current value of num1
        num2 = num2 * 3;          // Calculate the next value of num2
        printf("%d ", num2);      // Print the current value of num2
        num1 = num1 + 3;          // Calculate the next value of num1


    return 0;


  1. The variables num1 and num2 are initialized with 1 and 2, respectively.
  2. The for loop iterates 9 times (to print 9 terms).
  3. Inside the loop:
    • The current value of num1 is printed.
    • num2 is multiplied by 3 to obtain the next value.
    • The current value of num2 is printed.
    • num1 is incremented by 3 to obtain the next value.
  4. The updated values of num1 and num2 are used in the next iteration of the loop.

Key Points:

  • This approach effectively generates the specified number series using a combination of multiplication and addition operations.
  • The code is well-structured and easy to understand.
  • You can modify the value of n to control the number of terms printed.
  • Consider adding comments within the code to further enhance readability and maintainability.


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